1. Field of the Invention
The present invention relates to a high speed field effect transistor (FET) primarily used in a microwave band.
2. Related Background Art
An excellent high frequency characteristic is required for a semiconductor material of a FET used in a microwave band. Such a material includes an InGaAs semiconductor which exhibits a good electron transport characteristic. Among others, a high electron mobility transistor (HEMT, MODFET) which uses In.sub.0.52 Al.sub.0.48 As and In.sub.0.53 Ga.sub.0.47 As which match to InP in terms of lattice as an electron supply layer and a channel layer exhibits an excellent high frequency characteristic. Detail of the HEMT is described in IEEE Electron Device Letters, Vol. 11, No. 1, January 1990, pages 56 to 62. An AlGaAs/GaAs-HEMT which uses AlGaAs and GaAs semiconductor materials as the electron supply layer and the channel layer has been put into practice.
The semiconductor material for the electron supply layer is selected to have a larger energy gap and a smaller electron affinity than that of the semiconductor material for the channel layer. When the channel layer is made of GaAs, AlGaAs is selected as the material for the electron supply layer, and when the channel layer is made of InGaAs, AlInAs is selected as the material for the electron supply layer. Two-dimension electron gas is generated in the channel layer by forming the epitaxial structure of the HEMT by those materials.
Usually, a gate length of the FET is very short, for example less than 1 .mu.m. Thus, the electrons which run therealong have a large energy. Accordingly, when a high voltage is applied to a gate electrode, the electrons do not stay in the two-dimension channel but they may jump over an energy barrier which is present on a junction plane of the channel layer and the electron supply layer and are moved to the electron supply layer. This phenomenon is explained as follows with reference to an energy band chart of FIG. 1.
FIG. 1A shows an energy band on a hetero-junction plane of the AlGaAs/GaAs-HEMT; two-dimension electron gas shown by hatching is generated in the GaAs semiconductor near the junction interface. When a high voltage is applied to the gate electrode and a high electric field is applied to the electrons, the electrons e.sup.- in the GaAs semiconductor jump over the energy barrier which is present on the hetero-junction interface and are moved to the AlGaAs semiconductor. The same phenomenon takes place in the AlInAs/InGaAs-HEMT shown in FIG. 1B. When a high electric field is applied to the two-dimension electron gas generated in the InGaAs semiconductor and a high energy is imparted to the electrons e.sup.-, the electrons e.sup.- jump over the energy barrier on the hetero-junction interface and are moved to the AlInAs semiconductor.
The AlGaAs or AlInAs semiconductor material which forms the electron supply layer has a smaller electron mobility and a lower electron saturation speed than those of the GaAs or InGaAs semiconductor material which forms the channel layer. As a result, when a portion of electrons moves from the channel layer to the electron supply layer, the electron mobility of the entire current channel decreases and the electron saturation speed reduces. As a result, a high frequency characteristic of the FET is deteriorated. The deterioration of the characteristic is also seen in a transfer conductance g.sub.m which is an index to indicate a transfer function of the device. Namely, in a characteristic of transfer conductance g.sub.m vs gate voltage, the transfer conductance g.sub.m materially decreases as the gate voltage increases in a positive direction.
This problem is pointed out in IEEE Transactions on Electron Devices, Vol. ED-31, No. 1, January 1984 and it is noticed as a technical problem in the high speed FET.